The present application claims the benefit of priority from European Patent Application No. 01610079.4 filed on Jul. 16, 2001.
The present invention relates to voltage drop compensation in a multi-conductor cable connecting an electric power source with a consumer installation demanding supply of multi-phase AC power of a predetermined frequency and nominal voltage. In particular, the invention is concerned with the supply of ground power to aircraft standing in airport parking positions.
AC power supply systems including converter systems like UPS systems and Ground Power Units or GPU""s as used e.g. in airports for the supply of electric power to parked aircraft are frequently located far from the consumer installation. This can be due to environmental factors, hazards or space requirements. In an airport installation, a GPU is often mounted under the passenger bridge for connection with an aircraft through a cable in a hoist. An alternative approach may be location of the GPU in the airport building and running of the cable through a duct to a pit close to the aircraft parking position. Depending on the actual layout cable lengths of up to 100 meters may be required.
For the AC power supply demanded by an aircraft, typically 3xc3x97200 V, 400 Hz, the cable voltage drop is significant and the voltage quality at the aircraft installation may easily fail to comply with the voltage requirement of electrical loads on aircraft, specified as 115xc2x13 V in phase values. In addition, if unsymmetrical cables are used, significant unbalance will be generated at the output even with balanced loads.
The voltage drop may in practice be reduced by running several cables in parallel which is an undesirably expensive solution, however, requiring an excessive amount of copper far beyond what is required to reach a prescribed thermal capability of the cable.
By proper cable design the voltage drop can be somewhat reduced. Thus, with use of e.g. a 7 conductor symmetrical cable the inductive voltage drop will be balanced and reduced compared to a conventional unsymmetrical 4 conductor cable when applied with balanced load.
In some state of the art ground power units for airport use a possibility for compensation of the cable voltage drop is provided based on one of the following approaches:
1. Compensation based on one parameter, where the compensation is proportional to the amplitude of the GPU output current determined either as a mean value of the three phases or individually per phase. This form of compensation will only work with the specified power factor used during set-up. Because inductive load banks are not commonly available in airports, the compensation is normally adjusted to a power factor close to 1.0, whereas the power factor of aircraft loads is usually about 0.8. In result, by this approach correct voltage drop compensation can not be obtained.
2. Compensation based on two parameters, by which the converter is able to adjust the compensation depending on the power factor as well. With this approach, however, the effect of unsymmetrical cables can still not be compensated and with an unbalanced load the compensation voltage calculation will fail to include the additional voltage drop added to return current in the neutral conductor during unbalanced load conditions.
3. Feed-back of output voltages to the GPU through the control wire that is often included in power cables for 400 Hz airport installations. This approach is possible, if the control wires are twisted to cancel the coupling between the power conductors. By relying on feed-back from the load end of the cable the voltage control will be very dependent on the availability and reliability of the control wires. Since the control wires are much thinner than the power conductors, they show a tendency, however, to break over time, when the cable is bent during handling.
Both of approaches 1 and 2 require loading of the GPU and iterative adjustments which can be time consuming. Moreover, bad installation designs with use of unsymmetrical cables in a duct may require transposal of the cable wires in several places for compliance with output requirements.
Other approaches for line voltage drop compensation in electrical distribution systems have been disclosed.
Thus, in a line drop compensator device disclosed in U.S. Pat. No. 4,313,081 capacitance is added-between the conductor lines and the neutral of the power cable in response to the sensed current flow through the cables to correct the voltage in proportion to the actual demand. This requires current measurements at a plurality of service point along the three phase power line.
An electric power system with line drop compensation disclosed in U.S. Pat. No. 5,117,174 requires in addition to use of a local voltage regulator for monitoring the output voltage of the power source use of a remote voltage regulator for sensing voltage on the power bus at a point of regulation away from the power source to produce a PWM (Pulse Width Modulated) signal having a duty cycle representative of the voltage at the point of regulation.
In a power delivery system disclosed in U.S. Pat. No. 6,125,048 the voltage delivered from a central power unit is calibrated in accordance with the determined impedance of the supply lines, whereby the impedance determination can be provided by conduction of a reference voltage through a calibration line, transmission of a sweep tone down the supply line or a time domain reflectometer technique. Alternatively the impedance may be determined based on the measured input voltage to the remote unit supplied with power from the central unit or the measured power level of a signal transmitted from the remote unit.
On the background of this prior art it is the object of the invention to provide a method for compensation of voltage drop in a multi-conductor cable based on a novel compensation scheme involving the use of a stored cable model in the power source to produce voltage drop signals usable as reference produce voltage drop signals usable as reference signal for a voltage controller to provide a compensation voltage to be added to the nominal voltage level required by the consumer installation.
To meet this object the invention provides a method of compensating voltage drop in a multi-conductor cable connecting an electric power source with a consumer installation demanding supply of multi-phase AC power of a predetermined frequency and nominal voltage, in particular for supply of ground power to aircraft standing in airport parking positions, comprising adjustment of the output voltage of the electric power source to a level above said nominal voltage and further comprising the steps of
a) determining a set of cable impedance parameters,
b) storing said set of cable impedance parameters in the form of a cable model matrix in a memory forming part of a voltage drop compensation control circuit in the power source,
c) determining vector representations of the fundamental components of the individual output currents of the phases of the multi-phase AC power,
d) calculating for the phases of the multi-phase AC power a set of vector representations of fundamental voltage drop signals by matrix multiplication of the vector representations of said fundamental current components by said cable model matrix, and
e) using the vector representations of said fundamental voltage drop signals as reference signals for a voltage controller in said power source to produce for each of said phases a compensation voltage for addition to said nominal voltage.
By this novel compensation scheme based on the flux-linkage equations of the cable, the parameters of which are dependent on the physical distances of the cable conductors, voltage drop in symmetrical as well as unsymmetrical cables connected with balanced as well as unbalanced loads a significantly more accurate and well-functioning compensation of the cable voltage drop without any requirement for additional wires or conductors in the cable for calibration purposes or for additional equipment located at the consumer installation end or remote end of the cable.
In a preferred embodiment a complete set of cable impedance parameters for use in the calculation of the voltage drop signals can be determined in a very quick and accurate way by a method characterized in that said set of cable impedance parameters is determined by supplying identical AC currents to one of said conductors, returning in one single other conductor, and carrying out individual consecutive voltage drop measurements over said single conductor and said single other conductor, while a short-circuit is formed at the remote end of the cable.
In a particularly advantageous implementation of this embodiment consecutive voltage drop measurements are carried out by short-circuiting all conductors of the cable at the remote end with a plug, supplying identical AC currents to all of said conductors except a single other conductor at the cable end connected with the output of the power source, and measuring the voltage drop over said single one conductor and said single other conductor.
For implementation of the compensation method outlined above the invention further provides an electric power supply system including a power source comprising
a DC to AC inverter,
phase transformers connected with said DC to AC inverter for supply of multi-phase electrical power of a predetermined frequency and a nominal voltage to a consumer installation,
a voltage controller connected with said inverter for individual control of the voltage level of each phase produced thereby, and
a voltage drop compensation control circuit connected between an output of the power source and said voltage controller, said compensation control circuit comprising
a memory storing a cable model matrix including a set of cable impedance parameters,
means for determining individual phase currents at the output of said power source,
discrete Fourier transformation means for determining vector representations of the fundamental components of said individual phase currents,
matrix multiplication means for calculation for the phases of the multi-phase AC power a set of vector representations of fundamental voltage drop signals by matrix multiplication of the vector representations of said fundamental current components by said cable model matrix, and
means for supplying the vector representations of said fundamental voltage drop signals as reference signals to said voltage controller to produce for each of said phases a compensation voltage for addition to said nominal voltage.